1. Field of the Invention
The present invention relates to a plastic optical fiber having a polymer core constituted mainly of methyl methacrylate. Particularly, the invention is directed to a plastic optical fiber having high efficiency of guiding visible rays of 400 to 650 nm wavelengths and excellent performance of transmitting signals or optical energy.
2. Description of the Prior Art
For the purpose of producing a plastic optical fiber having high efficiency of light transmission, the primary object is to produce a core polymer having a high transparency. U.S. Pat. No. 4,161,500 discloses a process for spinning fiber which comprises the fractional distillation of a monomer in a sealed system, charging the refinded monomer through a filter having a pore size of 0.2 to 1 .mu.m into a cylindrical polymerization vessel having an inner diameter of 25 to 30 mm, sealing the vessel, completing the polymerization under specific pressure and temperature conditions, cooling and withdrawing the resulting solid preform, feeding it into the barrel of a ram extruder, and co-extruding the fed polymer as a core material together with a cladding material. It is reported that light attenuation through the thus produced plastic optical fiber was 274 dB/Km at 656 nm.
U.S. Pat. No. 4,381,269 proposes a polymerization process in a sealed system which comprises charging a monomer, polymerization initiator, and chain transfer agent through a distillation step into a polymerization vessel, bulk polymerizing the monomer to form a core polymer, and melt-spinning the obtained core polymer. In this process, the monomer is mixed with 0.1 mol % of an azo bis-t-butane polymerization initiator and 0.3 mol % of an n-butyl mercaptan chain transfer agent, is completely polymerized and the resulting polymer is extruded through a cock at the bottom to produce a plastic optical fiber. It is reported that light attenuations through this optical fiber are 90, 88, and 178 dB/Km at wavelengths of 523, 568, and 650 nm, respectively. In another example of this patent, a fiber is produced by similarly polymerizing a methyl methacrylate monomer using azo bis-t-butane and n-butyl mercaptan, heating the completely polymerized product to 200.degree. C., and extruding the polymer from the polymerization vessel by applying pressure with nitrogen gas. Light attenuations through the obtained optical fiber are confirmed to be 62, 58, and 130 dB/Km at wavelengths of 516, 566, and 648 nm, respectively. This patented invention is acceptable to the extent that it is the first to disclose that an attenuation of 100 dB/Km can be achieved with a plastic optical fiber, but the production process disclosed for producing this fiber involves problems when utilized for the manufacturer of plastic optical fibers which are utilizable in industrial applications.
In contrast to these processes for producing plastic optical fibers in sealed systems, U.S. Pat. No. 3,993,834 proposes a continuous bulk polymerization process for producing a core polymer, in which a reaction mixture of a monomer and--is continuously fed into a polymerization vessel, thoroughly stirred and kept at a temperature of above 130.degree. C. and below 160.degree. C. while maintaining polymer content .PHI. in said reaction mixture substantially constant, so as to satisfy the following relationship: EQU 50&lt;.PHI.&lt; exp (0.0121T-1.81)
wherein T represents the polymerization temperature in Calsius. Using the thus produced core polymer, a plastic optical fiber is fabricated.
Japanese patent application Laid-Open No. 104906/82 to proposes a process for producing a core polymer according to the continuous bulk polymerization technique of U.S. Pat. No. 3,993,834, except that the monomer, before being fed into a polymerization vessel, is filtered through a porous film. According to an example disclosed in this patent application, a light attenuation of 92 dB/Km at a wavelength of 577 nm is confirmed. Moreover, Japanese patent application Laid-Open No. 193502/83 proposes a continuous process for producing a plastic optical fiber which comprises successive removal of dissolved oxygen, monomer peroxide, and fine particles from a monomer, followed by continuous bulk polymerization of the purified monomer.
All the above stated prior techniques have been proposed to obtain high-performance plastic optical fibers, but none of these techniques produces plastic optical fiber which are satisfactory for practical use because each of these techniques are connected with the following various unsolved problems. For example, processes for producing plastic optical fibers in sealed systems, as proposed in U.S. Pat. Nos. 4,161,500 and 4,381,269, permit a high-degree purification of feedstock, but have the drawback in that when these processes are utilized, it is extremely difficult to clean the inner walls of the purification facilities and of the polymerization vessel to the same level as the level of the purified raw material. The cleaning of these systems is similarly or more important and more difficult than the cleaning of the raw materials. Since the stability of product quality and the economy of production are of extreme importance to an industrial production process, the cleaning of facilities becomes an issue in the case of the sealed systems wherein polymerization initiation is repeated each time and this is undesirable.
On the other hand, the continuous system is favorable for industrial production. However, when a monomer in the liquid state is filtered through separator films having pore sizes of 500 to 2000 .ANG. as described in Japanese patent application Laid-Open No. 104906/82, fine particles which remain in the monomer would have a significant effect so that a high-performance plastic optical fiber cannot be obtained. When a monomer in the vapor state is filtered, pores the of the filter tend to be clogged with polymeric matter so that a stable operation cannot be continued for long period of time. When methyl methacrylate or a monomer mixture composed mainly thereof is filtered through such ultrafilters with pore sizes of scores of angstroms capable of filtering off human albumin in a separation efficiency of at least 90% as described in Japanese patent application Laid-Open No. 193502/83 (filed by the present inventors), the polymer that formed therefrom increases with time passage and is caught by the filters, which gradually leads to their pores being clogged so that long-term continuous stable operation of such equipment is impossible. Further, in order to distill a monomer (methyl methacrylate or a monomer mixture composed mainly of it) in the absence of oxygen as described in Japanese patent application Laid-Open No. 193502/83, the monomer peroxide contaminating the monomer should be completely decomposed in advance by heat-treatment. Otherwise the distillate monomer will readily polymerize. In any case, difficulties in long-term operation are connected with the processes described above.